Generally (depends on the brand of the flush valve of course) one simply needs to shut off the water below the toilet. Then take the upper section off from the inlet valve. Hold a rag over that section of the valve (to catch the stream of water you are about to get), turn the water back on for a second. Then turn it back off, wash out the upper part that you took off and reinstall and then turn the water back on. IF, you did not get much water coming into the rag then you know the source of the problem is BEFORE the valve.